One-time programmable (OTP) memories having antifuse memory cells are programmed using voltage or current pulses. Each antifuse memory cell contains an insulator such as a gate oxide that is supplied in a high-impedance (non-conducting) state. Upon application of a programming pulse, the insulator breaks down such that it is irreversibly transformed into a low-impedance (conducting) state. As compared to other types of OTP memories such as the charge storage approach used in OTP flash, antifuses are more reliable in that charge leakage will occur over time in OTP flash. This charge leakage is exacerbated as circuit dimensions are made ever smaller with the advance of semiconductor manufacturing technology.
Although antifuse memories do not suffer from flash charge leakage, reliability problems exist with these memories as well. For example, as circuit dimensions continue to shrink, the insulator in the antifuse memory cells such as gate oxide has its thickness reduced correspondingly such that electrons may quantum mechanically tunnel through the insulating layer. Because electrons are tunneling rather than conducting through the insulating layer, “burning” the insulating layer so as to program antifuse memory cells becomes increasingly difficult. Thus, users of antifuse memory cells encounter increased difficulties with reliably breaking down the insulating layer to a predictable conductivity. Moreover, even un-programmed antifuse memory cells can conduct appreciable leakage currents such that leaky un-programmed antifuse cells are incorrectly detected as having been programmed to a conducting state.
Accordingly, there is a need in the art for antifuse memory cells having improved reliability.